I have built the HPPA (High Performance Power Amplifier) a couple of years ago. I later sold it to a friend of mine who still uses it. The amplifier sound quality was pretty good, but I haven't really compared it to other amplifiers. One thing I do remember is that it was quite hard to set the bias current. The bias current increased as the temperature of the heatsink increased... It was impossible to get the bias current stable at some currents. I mounted the six power transistors on a 5mm al U-profile, which was bolted onto the heatsink. And I also used a temperature controlled fan.

The designer of this amp is Bert Molenkamp.

Here is some info I've got:
Notes on how to assemble/build the HPPA.

Use a good quality double sided PCB.
The component side is used as ground surface.

The PCB's are 300 dpi images. The exact dimensions of the PCB should be 151 * 150 mm, for all parts to
fit well. (6 * 6 inch)

There are a few differences between the schematic amd the actual PCB-lay-out.
Instead of the BD791/792, I normally use a BD139/140 pair. These will do fine under normal conditions
(ambient tempreatures lower than 40 degrees Celcius).
For the low-power transistors I use the BC556 instead of BC560's. The Uce of the BC556 is higher,
which is required with high supply voltages. But beware: the connections of the BC556 are different from
the one shown in the component lay-out.
When using the amp as a low power class A amp, with lower PSU voltages, use the BC560 for better
noise preformance.

Don't forget to solder the components, which are connected to ground, on both sides of the PCB!

- On the solderside of the PCB, connenct a 47R resistance (2Watt) between the emitters of the
TIP142/147 darlingtons (the value of this resistor depends on the quiescent current you want to use, but
for a start 47R might be the best choice)
For best performance, parallel this resistor with a 1u5 polypropylene capacitor.

- The four emitter resistors of the output transistors should be connected to each other with a short, very
low resistance cable ( I usually use four 6cm pieces of Monster Cable (6mm); two twisted pairs, one side
soldered to the emitter-resistor pads, the four wire ends twisted and connected to the output-relais)

- The two 1k resistors of the current-source on the lower left corner of the PCB, should NOT be soldered
to the solder side. Instead, they should be connected to a 7824 voltage regulator, as shown in the
schematic diagram. I usually build this 'in the air'.

- The missing components near the 0R22 resistors, form a short-circuit protection. I never use this for
home-purposes, it is said to affect the sound in a negative way.

The two BD140's and the two BD139's in the middle of the PCB must be mounted to the same heatsink.
This must be large enough, especially when using high supply voltages.

The six output transistors should be mounted on the same, very large heatsink. See the picture on how to
get the best results. When running the amp in class A, it's wise to use forced cooling with a low noise fan.

The amp will perform better when the MJ3281A/MJ1302A combination is used. This reduces output
power a bit, as the maximum Ic is lower, but increases the slewing rate enormously. The fT of this pair is
30 Mhz, which is remarkable for transistors of this power-rating.
The darlington drivers are the limiting factor in this case, but switching from TIP142/147 to
MJH6284/6287 would be the right thing to consider.
In case of a low power class A amp, the darlingtons could be replaced by medium power transistors with a
high fT and higher hFE, like the MJE15030/15031.

For all heatsinks, use aluminum of at least 4mm thick, but for the output transistors it might be a good
idea to use 6mm instead.

hugobross

30th July 2002 02:32 PM

I know somebody who made that amp too, it seems to work very well. I've also made an amp (much simpler in design than yours) with the TIP142/147 pair, but my experiences with them is that they're to "weak" to use them as output devices, use them in combination with MJ15003/MJ15004 like in your schematic.

My amp worked fine with only one pair of TIP142/147, but the safety margin is too low. I'm still doing some modifications to the design, see: